Group III-nitride semiconductors have seen enormous commercial growth in recent years for solid state lighting and power electronics. While the majority of light emitting diodes (LEDs) are grown by metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) has seen success in the power electronics market for the growth of high electron mobility transistors (HEMTs)) and promises expansion of light emitters to wavelengths currently challenging for MOCVD. Plasma-assisted MBE (PAMBE) typically uses radio frequency (RF) plasma to generate reactive nitrogen species from inert nitrogen gas. PAMBE using RF plasma has been shown to result in higher growth rates and improved surface morphology compared to other plasma techniques owing to lower ion content and reduced surface damage during growth.
Nonetheless, PAMBE is typically performed at growth rates on the order of 0.1 to 1 μm/hour, substantially lower than the growth rates typically used in MOCVD growth of GaN which commonly exceed 1-3 μm/hour. These relatively low growth rates limit the applicability of PAMBE for many device structures which require thick buffer layers to reduce defect densities resulting from heteroepitaxy. Although the ultra clean environment of MBE can be beneficial for the thick, undoped drift regions of p-i-n rectifiers or LEDs, the slow growth rates still preclude the use of MBE for such devices. Finally, if III-Nitrides, with their tunable band gap and strong light absorption, are ever to become viable for solar applications, thick indium-bearing layers must be demonstrated in order to reduce defect densities. Such layers with high indium mole fractions are impractical in MOCVD due to the low temperature requirements for growing indium bearing alloys, while high temperatures are required for cracking ammonia. The result for most MOCVD growth of InGaN is very low growth rates and highly inefficient precursor usage. Contrarily, these high indium composition alloys are dramatically more suitable for PAMBE where substrate temperatures can be reduced in order to facilitate proper indium incorporation and no ammonia cracking is required. If rapid growth of indium-bearing layers can be achieved, then the potential of III-nitrides for photovoltaics improves dramatically.